This invention relates to a marine propulsion system, and more particularly to such a system incorporating dual counterrotating propellers.
It is known to provide dual counterrotating propellers in a marine propulsion system in order to reduce or eliminate hydrodynamics inefficiencies present in a single propeller system. For example, it is known that, in a single propeller system, torque loads imposed by the rotating single propeller must be overcome in order to effect steering of the system. Utilization of dual counterrotating propellers substantially reduces or eliminates such torque loads.
It has been found that employing a chain drive mechanism for driving the propeller in a marine propulsion system results in improved operation of the system. One such system is disclosed in copending application Ser. No. 07/224,994 filed Sept. 15, 1988, and entitled "Stern Drive Marine Propulsion System Including A Chain Drive Mechanism". Further, it has been found that a chain drive mechanism can advantageously be employed in a dual counterrotating propeller system. One such system is disclosed in copending application Ser. No. 07/224,994 filed Sept. 9, 1988, and entitled "Chain Drive Marine Propulsion System With Dual Counterrotating Propellers".
A gearcase including a steerable lower portion has proven effective in certain applications. One such steerable gearcase structure is disclosed in copending application Ser. No. 07/253,046 filed Oct. 4, 1988, and entitled "Gear Driven Marine Propulsion Systems With Steerable Gearcase And Dual Counterrotating Propellers".
The present invention is intended to achieve the benefits of a chain drive Counterrotating propeller system, in combination with advantages offered by a steerable gearcase structure. In accordance with the invention, a marine drive including an engine having a rotatable output shaft is provided with a depending gearcase having an upper portion and a lower steerable portion. The lower steerable portion is mounted to the upper gearcase portion so as to be pivotable about a steering axis. First and second propeller shafts are rotatably mounted to the lower steerable portion of the gearcase, and first and second propellers are mounted to the first and second propeller shafts, respectively. Chain means is drivingly interconnected between the engine output shaft and the first and second propeller shafts. The chain means includes a first portion for driving the first propeller shaft and a second portion for driving the second propeller shaft. Counterrotation drive means is provided for driving the first and second portions of the chain means in opposite directions in response to rotation of the engine output shaft. Accordingly, counterrotation of the first and second propeller shafts and the first and second propellers is thereby provided. The counterrotation drive means accommodates pivoting movement of the lower steerable gearcase portion about the steering axis. In one embodiment, the first and second propeller shafts are disposed one within the other, and are substantially coaxial. The first and second portions of the chain means extend between the first and second propeller shafts and a pair of rotatable intermediate drive members, with the counterrotation drive means providing rotation of the intermediate drive members in opposite rotatational directions. The intermediate drive members are preferably disposed above the water line during boat operation, thereby reducing the frontal area of the submerged portion of the steerable gearcase. The counterrotation drive means preferably comprises a pair of coaxially extending shafts interconnected with the engine output shaft so as to rotate in opposite directions in response to rotation of the engine output shaft. The longitudinal axis of the coaxial shafts preferably defines the steering axis about which the lower steerable gearcase portion is pivotable. In the disclosed embodiment, gear means is provided between the engine output shaft and the coaxial shafts for providing counterrotation thereof. The coaxial shafts are preferably interconnected with the intermediate driving members through appropriate gearing so as to provide counterrotation thereof.
While the invention is applicable to either an outboard or inboard/outboard stern drive system, a stern drive embodiment of the invention is disclosed. The steerable gearcase portion is provided with an upwardly extending projection which is rotatably mounted to the upper gearcase portion by means of bearings or the like. In this manner, the lower gearcase portion is suspended from the upper gearcase portion. A rack and pinion mechanism is provided in the upper gearcase portion for effecting pivoting movement of the upwardly extending projection of the lower gearcase portion about the steering axis.